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From you host … Dr. H

Lecture 6: Telecommunications. From you host … Dr. H. Introduction. “A communications architecture is the arrangement, or configuration, of satellites and ground stations in a space system, and the network of communication links that transfer information between them.” L & W, SMAD.

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From you host … Dr. H

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  1. Lecture 6: Telecommunications From you host … Dr. H

  2. Introduction “A communications architecture is the arrangement, or configuration, of satellites and ground stations in a space system, and the network of communication links that transfer information between them.” L & W, SMAD • Communications design requires us to think about the following issues: • Over what time durations and over what distances does the spacecraft need to communicate with Earth? • What does the spacecraft need to communicate & how often? • How much command, control and decision making is local and howmuch is centered on Earth?

  3. Steps in Defining a Communications Architecture • Identify communication links • Define mission objectives • Define mission requirements • Determine the architecture • Determine data rates for each link • Specify accuracy required • Determine sampling rates, quantization levels. • Design each link • Select frequency band • Select modulation, coding • Determine antenna size, beamwidth constraints • Determine transmitter power constraints • Estimate received noise, interference powers • Calculate antenna gains & transmitter power • Size the comm system • Select antenna configuration • Calculate antenna size • Estimate antenna masses • Estimate transmitter masses

  4. Comm. Architecture Defined by Function Spacecraft Relay Spacecraft

  5. Step 2: Determine Data Rates for Each Link • What information must be communicated and how fast? • Analog-to-digital conversion: • Sample frequency >2.2 x (max input frequency) • Divide the range of the analog signal into M =2n levels – n = no. of bits per sample • Mean-square noise power = (V)2/12, where V = Vfull-scale/M • Signal-to-quantization noise power ratio = (M2-1). So you need smaller steps for weaker signals.

  6. Step 2: Determine Data Rates • No. of bits per sample determined by mission requirements • Data rate = (No. samples/sec.) X (No. of bits/sample)Abbreviated: bps

  7. Step 2: Data Rates for TT&C • Monitoring: • Several hundred functions might be sampled, but at a low rate • Typical data rate ~ 50bps • Transmitting commands: • Usually ~ 1/sec. • Command message is typically ~ 48 to 64 bits • Tracking: • Ground station measures range or range rate for computing orbit ephemeris • For typical parameters of existing TT&C systems see table on next slide

  8. Step 2: Existing TT&C Systems

  9. Step 2: Data Rates for Data Collection • Example: A geostationary satellite with a radiometer which scans the entire Earth in 20 min. with 1 km resolution

  10. Step 2: Data Rates for Data Relay • Data relay systems typically re-transmit data with a receiver/transmitter combination called a transponder • Transponder bandwidths of commercial communication satellites are usually 36 MHz or 72 MHz. • Maximum data rate can be several times the bandwidth, depending on the modulation and the receiving station size

  11. r Step 3: Link Design~Electromagnetic Signal Propagation~ • Isotropic antenna: • Point source with total power, P • Total power flowing through any spherical surface of radius r remains = P • Therefore, power per unit area flowing to a receiver distance r away is proportional to 1/r2

  12. Step 3: Link Design~Electromagnetic Signal Propagation~ • But real transmit antennas have directionality. • They radiate preferentially toward the axial direction • As a result, the power per unit area transmitted along the axis is: • Gt x (power radiated by an isotropic antenna) • And this defines the transmitting antenna gain, Gt

  13. Noise r P P =transmitter power Ll = transm.-to-anten. line loss x Gt LsLa Gt = transm, anten. Gain Ls = space loss =(/4r)2 La = transm. path loss x Gr Ar Gr = receive anten. Gain Ar= effective receive antenna area Step 3: Link Design

  14. Step 3: Link Design – Link Equation Derivation

  15. Step 3: Link Equation Derivation - Continued

  16. Step 3: Link Design Equations in dB

  17. Step 3: Link Design Equations in dBTypical Noise Temperatures In s/c Comm Links

  18. Step 3: Detailed Procedure for Link Design

  19. Step 3: Detailed Procedure for Link Design- Continued

  20. Step 4: Sizing the Communications SystemEstimated transmit power + aperture size  System mass

  21. FINI

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